Sébastien R. Mouchet

Theoretical condition for transparency in mesoporous layered optical media: application to switching of hygrochromic coatings

Mesoporous Bragg stacks are able to change color upon infiltration or displacement of liquid compounds inside their porous structure. Reversible switching from transparency to coloration offers additional functionality. Based on Bruggemans effective medium theory, we derive a transparency master equation, which is valid for bilayers of arbitrary host materials and pore-filling compounds. The transparency condition fixes pore volume fractions such that the effective refractive index is homogenized through the bilayer, hence, through arbitrary layered optical media built from this bilayer. This general concept is applied to the case of switching of hygrochromic coatings made of mesoporous mixed oxide Bragg stacks.

Liquid-induced colour change in a beetle: the concept of a photonic cell.

The structural colour of male Hoplia coerulea beetles is notable for changing from blue to green upon contact with water. In fact, reversible changes in both colour and fluorescence are induced in this beetle by various liquids, although the mechanism has never been fully explained. Changes enacted by water are much faster than those by ethanol, in spite of ethanol’s more rapid spread across the elytral surface. Moreover, the beetle’s photonic structure is enclosed by a thin scale envelope preventing direct contact with the liquid. Here, we note the presence of sodium, potassium and calcium salts in the scale material that mediate the penetration of liquid through putative micropores. The result leads to the novel concept of a “photonic cell”: namely, a biocompatible photonic structure that is encased by a permeable envelope which mediates liquid-induced colour changes in that photonic structure. Engineered photonic cells dispersed in culture media could revolutionize the monitoring of cell-metabolism.

Vapor sensing with a natural photonic cell.

Photonic structures encased by a permeable envelope give rise to iridescent blue color in the scales covering the male Hoplia coerulea beetle. This structure comprises a periodic porous multilayer. The color of these scales is known for changing from blue to green upon contact with water despite the presence of the envelope. This optical system has been referred to as a photonic cell due to the role of the envelope that mediates fluid exchanges with the surrounding environment. Following from previously studied liquid-induced changes in the color appearance of H. coerulea, we measured vapor-induced color changes in its appearance. This response to vapor exposure was marked by reflectance redshift and an increase in peak reflectance intensity. Different physico-chemical processes were investigated to explain the increase in reflectance intensity, a property not usually associated with vapor-induced optical signature changes. These simulations indicated the optical response arose from physisorption of a liquid film on the beetle scales followed by liquid penetration through the envelope and the filling of micropores within the body of the photonic structure.

Vapour sensitivity of an ALD hierarchical photonic structure inspired by Morpho

The unique architecture of iridescent Morpho butterfly scales is known to exhibit different optical responses to various vapours. However, the mechanism behind this phenomenon is not fully quantitatively understood. This work reports on process developments in the micro-fabrication of a Morpho-inspired photonic structure in atomic layer deposited (ALD) materials in order to investigate the vapour optical sensitivity of such artificial nanostructures. By developing recipes for dry and wet etching of ALD oxides, we micro-fabricated two structures: one combining Al2O3 and TiO2, and the other combining Al2O3 and HfO2. For the first time, we report the optical response of such ALD Morpho-like structures measured under a controlled flow of either ethanol or isopropyl alcohol (IPA) vapour. In spite of the small magnitude of the effect, the results show a selective vapour response (depending on the materials used).

Hoplia coerulea, a porous natural photonic structure as template of optical vapour sensor

Natural photonic structures found on the cuticle of insects are known to give rise to astonishing structural colours. These ordered porous structures are made of biopolymers, such as chitin, and some of them possess the property to change colour according to the surrounding atmosphere composition. This phenomenon is still not completely understood. We investigated the structure found on the cuticle of the male beetle Hoplia coerulea (Scarabaeidae). The structure, in this case, consists in a 1D periodic porous multilayer inside scales, reflecting incident light in the blue. The colour variations were quantified by reflectance spectral measurements using water, ethanol and acetone vapours. A 1D scattering matrix formalism was used for modelling light reflection on the photonic multilayer. The origin of the reported colour changes has to be tracked in variations of the effective refractive index and of the photonic structure dimensions. This remarkable phenomenon observed for a non-open but still porous multilayer could be very interesting for vapour sensing applications and smart glass windows.

Controlled fluorescence in a beetle’s photonic structure and its sensitivity to environmentally induced changes

The scales covering the elytra of the male Hoplia coerulea beetle contain fluorophores embedded within a porous photonic structure. The photonic structure controls both insect colour (reflected light) and fluorescence emission. Herein, the effects of water-induced changes on the fluorescence emission from the beetle were investigated. The fluorescence emission peak wavelength was observed to blue-shift on water immersion of the elytra whereas its reflectance peak wavelength was observed to red-shift. Time-resolved fluorescence measurements, together with optical simulations, confirmed that the radiative emission is controlled by a naturally engineered photonic bandgap while the elytra are in the dry state, whereas non-radiative relaxation pathways dominate the emission response of wet elytra.

Method for modeling additive color effect in photonic polycrystals with form anisotropic elements: the case of Entimus imperialis weevil

The calculation of the reflectance of photonic crystals having form-birefringent anisotropic elements in the crystal unit cell, such as cylinders, often turns out to be problematic, especially when the reflectance spectrum has to be computed according to different crystal orientations as in polycrystals for instance. The method we propose here solves this problem in the specific case of photonic crystals whose periodicities are such that there are no diffraction orders except Bragg reflection in the visible range. For a given crystal orientation, the crystal is sliced into layers and the periodic spatial variations of the dielectric function ε are homogenized. Thanks to that homogenization, the calculation can be performed using standard thin film computation codes. In order to demonstrate the usefulness of our method, we applied it to the case of a natural photonic polycrystal found on the cuticle of Entimus imperialis weevil which is a remarkable example of additive color effect. Although each photonic crystal grain of the polycrystal produces a single bright iridescent color, a non-iridescent green matt coloration is perceived by the human eye due to multiscale averaging effects.

Linear and Nonlinear Optical Effects in Biophotonic Structures using Classical and Nonclassical Light

In this perspective article, we review the optical study of different biophotonic geometries and biological structures using classical light in linear and nonlinear regime, especially highlighting the link between these morphologies and modern biomedical research. Additionally, the importance of nonlinear optical study in biological research, beyond traditional cell imaging is also highlighted and described. Finally, we present a short introduction regarding nonclassical light and describe the new future perspective of quantum optical study in biology, revealing the link between quantum realm and biological research.

Modeling Additive Color Effect in Natural Photonic Polycrystals Using the Layer Homogenization Method: The Case of the Diamond Weevil

The original method we propose allows to compute the spectral reflectance of photonic crystals whose unit cell is composed of form-birefringence anisotropic elements such as cylinders or parallelepipeds. This method relies on the layer homogenization of the photonic structure. It is especially useful for the calculation of reflectance according to different crystal orientations. The coloration due to an additive color effect in the photonic polycrystal found on the diamond weevil, Entimus imperialis, was investigated. Simulating the reflectance of photonic polycrystals often turns out to be necessary in the study of such structures as well as in the design and production of bioinspired devices (Vigneron and Lousse, Proc SPIE 6128:61281G, 2006; Deparis and Vigneron, Mater Sci Eng B-Adv 169(1–3):12–15, 2010). In this context, the computation of the reflectance of photonic crystals (PCs) displaying form-birefringence anisotropic elements in the unit cell (e.g., cylinders, parallelepipeds…) turns out to be cumbersome, particularly when the reflectance is calculated for different crystal orientations as in the case of polycrystals. The method proposed here solves this problem in the particular case of a PC with a periodicity that is such that there is only specular reflection and no higher-order diffraction (Mouchet et al. Opt Express 21(11):13228–13240, 2013). In this method, the structure with a particular crystal orientation is sliced into layers and the periodic dielectric function \(\varepsilon \left (\overrightarrow{r}\right )\) is homogenized within each layer (Fig. 60.1a). Using this Layer Homogenization (LH) method, the reflectance of one single domain of the polycrystal can be computed in an arbitrary orientation thanks to a standard thin film solver. The reflectance due to the additive color effect created by the disorder in the crystal domain orientation of the polycrystal is modeled by averaging reflectance spectra computed for several incidence angles and crystal domain orientations. Our method was applied to the case of a natural photonic polycrystal found on the cuticle of the diamond weevil Entimus imperialis (Fig. 60.2a). Its coloration is due to an additive color effect created by PC domains with various orientations (Fig. 60.2b) (Deparis and Vigneron, Mater Sci Eng B-Adv 169(1–3):12–15, 2010; Mouchet et al. Proc SPIE 8480:848003, 2012): a single PC domain gives rise to a gleaming iridescent color (from blue to orange) but the disorder in the crystal domain orientation results in a non-iridescent dull color (Fig. 60.2a). Investigating such a structure is relevant in the development of bioinspired applications such as biomimetic devices e.g., gas, temperature or pH sensors (Van Opdenbosch et al. Photon Nanostruct 10(4):516–522, 2012).